In the old AD&D days, you received Experience Points (XP) for killing monsters and acquiring loot (as in, 1gp = 1XP) – but in our campaign it’s more about how you play. It’s important to run your character in a way that is consistent with the character’s definition: alignment, personality, personal goals, etc.: in the words of Mr. Glass,
“… doing what you’re supposed to be doing.”
Gaining XP could be seen as somewhat analogous to the process of individuation in Jungian psychology or Maslow’s idea of self-actualization: it’s about acting in such a way as to become what you’re meant to be, to fulfill your character’s “destiny” …
For example, let’s say you’re a Lawful Good Paladin: For you, the killing of a foul, evil creature is probably worthy of XP – but if you kill a creature that doesn’t deserve death, or you do it in a dishonorable manner (like stabbing it in the back) then you can expect to lose XP.
Experience Points are awarded for a variety of achievements (if achieved in the proper manner): including finishing missions, solving problems … and, yes, killing monsters and finding treasure (especially if violence and greed, respectively, are consistent with your character’s idiom). Totals for each player character are tracked by the DM – players will be notified when they level-up.
Refer to alignment and motivation for more details.
See also Leveling-Up under House Rules.